The art of printing has been transformed by the advent of digital printers. A digital printer may be described as a printing device which responds to digital signals. Digital printers and their associated equipment allow for the convenient preparation and storage of data for printing at the time of preparation or for later printing. Digital printers include a broad range of devices, from a small desk-top ink jet printer to elaborate, high speed, web-fed commercial printers.
An advantage of digital printing is that a separate physical medium is not needed to transfer the printing indicia or ink from the medium to the surface that is being printed, e.g., paper or plastic. Thus, in rotogravure or flexographic printing, plates are required to physically transfer the ink from the ink source to the paper or plastic. These plates are very expensive to make, and once made, must be stored and maintained for future runs. Once made, the plates cannot be changed. Thus, non-digital printing options tend to be expensive and also require high runs, many thousands, such as 50,000 or more, in order to be economical.
Digital printing can overcome these disadvantages in cost and convenience, but there can be difficulties, principally in adhering the ink to the printed substrate. Polyvinyl chloride (PVC) is one of the few plastics that are easily printed with conventional inks. Inks do not so easily adhere to other substrates. For example, U.S. Pat. No. 6,951,377 uses an ozonation process to raise the surface energy of a plastic substrate so that ink can more easily adhere to the plastic. This process allows the ink to more easily adhere, but also affects the physical and melting properties of the plastic. In addition, ozone may be a harmful pollutant and people working nearby should be protected from exposure to high ozone levels.
It is clear that ink will more easily adhere to substrates that are absorbent, such as paper or fiber, rather than less expensive plastic. One alternative is to use more expensive layers of plastic, as described in U.S. Pat. No. 6,793,859. This patent describes a printing substrate made with a core layer of polyester, polycarbonate, or other resin, and an image receptive layer. The image-receptive layer is made by extruding a carrier resin with an ink absorptive resin. The ink absorptive resin is preferably a copolymer of methylmethacrylate and other acrylates. This is an not only an expensive process, but also a time-consuming specialty chemical process as well.
U.S. Pat. Appl. Publ. 2003/0173716 also supplies a substrate that more easily accepts an ink layer. This application forms a better substrate by blending a coloring agent with a polyolefin film, which is then extruded and oriented with conventional extrusion and orientation equipment. The resulting film may be made in as many layers of as many colors as desired. Making and storing rolls of film by this process will require a large inventory to meet customer demand for the particular desired colors. In addition, the requirement for blending, extruded, and orienting the material adds to the cost of the process.
An alternative to these processes is to acquire or print a roll of film in the desired color and to print the roll or a large quantity of items with some of the desired features in the desired colors, such as a background or sales logo. When the need arises, the remainder of the desired printing, such as information concerning the dates of a particular sale, or the particular items offered, may be printed in the desired small quantity. This technique could require carrying a substantial inventory of pre-printed rolls, and also requires two printing operations rather than one.
There is a substantial need for color printed materials in relatively small volumes. As noted, conventional printing can satisfy the needs of customers with demands for high volumes of material, such as 50,000 copies or more. What is needed is a better way to digitally print small quantities without the need for extensive processing that alters the basic physical qualities of the material that is being printed, and without the need for a second round of printing.